Fix comments that claimed that mblen() only looks at first byte.

Fix comments that claimed that mblen() only looks at first byte.

GB18030's mblen() function looks at the first and the second byte of the
multibyte character, to determine its length. copy.c had made the
assumption that mblen() only looks at the first byte, but it turns out to
work out fine, because of the way the GB18030 encoding works. COPY will
see a 4-byte encoded character as two 2-byte encoded characters, which is
enough for COPY's purposes. It cannot mix those up with delimiter or
escaping characters, because only single-byte ASCII characters are
supported as delimiters or escape characters.

Discussion: https://www.postgresql.org/message-id/7704d099-9643-2a55-fb0e-becd64400dcb%40iki.fi